Step screens of the above type are known, for instance, from SE-B 461,284 and are commonly used to separate solid particles and other objects from municipal and industrial wastewater.
The fixed and movable lamellar rods have usually a thickness in the order of 2 to 6 mm, while the intermediate gaps between the fixed and movable lamellar rods to allow the flowing water to pass can normally amount to between 1 and 6 mm. In operation, as the movable lamellar rods move upwards between the fixed lamellar rods, ducts will be formed between the fixed lamellar rods at their lowermost step, the width of the ducts substantially corresponding to the thickness of the respective movable lamellar rods and twice the gap size, i.e. from about 4 to about 18 mm.
Through these ducts, correspondingly large particles and objects can pass more or less unimpededly owing to the relatively high speed of the flowing water and be entrained the flowing water. As a result, the water will not be sufficiently screened, thus requiring additional screening and/or causing problems in the subsequent treatment of the water.
Alternatively, excessively large particles and objects get stuck in the ducts and are hit by the lower edges of the movable lamellar rods when these movable lamellar rods after the upward motion again move downwards between the fixed lamellar rods and restore normal gap sizes. Consequently, the lower ends of the movable lamellar rods can be damaged, and finally the entire step screen may stop functioning.
FIG. 7 in the above-mentioned SE B 461,284 shows an attempt to reduce the above-described problems by a design where a covering member in the form of a plate is articulated with its one, first end to the lowermost steps of the movable lamellar rods and is designed to extend with its opposite, second end towards the bottom of a gutter for the flowing water, preferably to slide towards the bottom of the gutter as the lamellar rods move.
However, this design does not solve the problems above since the plate certainly covers the edges upstream on the lowermost step of the fixed lamellar rods but not the upper side of these steps, when the movable lamellar rods move from their lower to their upper position in their closed, circular motion path. On the upper side of the lowermost steps of the fixed lamellar rods, the above-described ducts are positioned, through which solid particles and other objects can pass or in which they get stuck, with the ensuing problems as described.